Various methods have been considered for suitably implementing positioning of the multicore optical fiber in which a plurality of cores each extending along a predetermined axis were covered by a common cladding. For example, Patent Literature 1 discloses the technology of processing a part of a cross-sectional shape of a preform into a flat shape and thereafter drawing the processed preform to manufacture the MCF with a cross section of a non-circular shape. It shows a method with this configuration to readily implement rotational alignment to a predetermined angular direction around a predetermined axis extending along the longitudinal direction of the MCF. Patent Literature 2 describes the MCF provided with a positioning structure wherein this positioning structure restricts rotation of the MCF around the predetermined axis extending along the longitudinal direction of the MCF. Furthermore, Non Patent Literature 1 describes the MCF in which four single-core optical fibers are integrated and the cross-sectional shape of which is non-circular.